No wonder it all fell down when I removed that window: the roof was several layers thick, and waterlogged, and therefore very heavy. Had it fallen down on me and the dogs when we were sheltering from a rain shower, injuries would have been likely.
There was a layer of wood planks, all on the road to rottenness. They were covered by 2 sheets of overlapping tin, laid in a way that water was going to percolate through. Then a partial layer of sodden chipboard, so degraded that there were at least a couple of brandling worms living in it - how they got there, I cannot imagine.
Another layer of tin, one single sheet this time, nailed down. Then two layers of roofing felt. "Tarry toosh", my Dad always calls it, I'm uncertain of the spelling: toosh rhymes with whoosh. Googled it just now with no helpful result.
Finally, there was a layer of various sheets of chipboard and what-not, which had likely been put on the roof out of the way, and rotted down, adhering to the felt. The adjoining potting-shed roof was plastic, wood and roofing felt, and a rubbery sheet of some kind.
Every single scrap of wood in the entire shed, bar one old door, is rotten and useless. I can recycle the tin sheets in the new shed, I hope. I now have 2 doors, and plenty of tin sheeting to improvise a rough tool chest. I'll keep a golf umbrella there, too, rainshowers, for use during.
You can see in the photo to the right the corner of a substantial bonfire pile. Which is a whole 'nother blog post.